1. Field of the Invention
The present invention relates to a display device, in particular, a display device including a plurality of thin-film transistors formed on a substrate on which a display area is formed.
2. Description of the Related Art
A display device includes a large number of pixels formed in its display area and arranged in matrix. The pixels are normally driven by a so-called active matrix method.
That is, pixel groups each including a plurality of pixels arranged in a row direction are sequentially selected, and when a group of pixels are selected, a video signal is fed to the selected group of pixels via a drain signal line connected commonly to pixels arranged in a column direction. In this case, each group of pixels is selected by turning on thin-film transistors, which are respectively formed in the pixels constituting the group of pixels, with a scanning signal fed to the thin-film transistors via a gate signal line that is connected commonly to the thin-film transistors.
A typical one of such display devices includes a drain driver that feeds a video signal to each drain signal line, and a gate driver that feeds a scanning signal to each gate signal line. The drivers are formed on a substrate and in a periphery of a display area formed on the substrate. Each driver includes a large number of thin-film transistors formed in parallel with the thin-film transistors formed in the pixels.
A so-called bottom-gate type thin-film transistor is known as an example of such thin-film transistors. In the bottom-gate type thin-film transistor, an interlayer insulating film with an opening is formed on an upper surface of a gate insulating film, and a polysilicon layer is formed to cover the opening (see Japanese Patent Application Laid-open No. 2004-193248 and Japanese Patent Application Laid-open No. Hei 11-186558). In the thin-film transistor configured accordingly, a portion of the polysilicon layer that is formed at the bottom of the opening functions as a channel area, and a portion of the polysilicon layer that is formed on the side wall surface of the opening serves as an offset area. Therefore, the electric field at the end of the drain may be limited to reduce an off current.
The details of the structure of the thin-film transistor disclosed in Japanese Patent Application Laid-open No. 2004-193248 are as follows. An interlayer insulating film is formed on the upper surface of a gate insulating film, under which a gate electrode is formed. The interlayer insulating film has an opening generally at the center of an area overlapping the gate electrode in plan view. A drain electrode and a source electrode are formed on the upper surface of the interlayer insulating film with the opening interposed therebetween. Each of the drain electrode and the source electrode is formed of a laminate obtained by sequentially laminating a heavily-doped amorphous silicon layer and a metal layer. A polysilicon layer is formed to cover the opening of the interlayer insulating film such that a peripheral portion of the polysilicon layer overlaps a part of each of the drain electrode and the source electrode.
The details of the structure of the thin-film transistor disclosed in Japanese Patent Application Laid-open No. Hei 11-186558 are as follows. An interlayer insulating film is formed on the upper surface of a gate insulating film, under which a gate electrode is formed. The interlayer insulating film has an opening in an area overlapping the gate electrode in plan view. A polysilicon layer is formed to cover the opening of the interlayer insulating film. The polysilicon layer has a drain area and a source area formed on the upper surface of the interlayer insulating film. The drain area and the source area are formed by ion implantation of high-concentration impurities into the semiconductor film. A source electrode is formed between the gate insulating film and the interlayer insulating film. The source electrode and the source area of the polysilicon layer are connected through a contact hole formed in the interlayer insulating film.